The East China Sea is an arm of the Western Pacific Ocean, located directly offshore from East China. It covers an area of roughly 1,249,000 square kilometers (482,000 sq mi). The sea’s northern extension between mainland China and the Korean Peninsula is the Yellow Sea, separated by an imaginary line between the eastern tip of Qidong at the Yangtze River estuary and the southwestern tip of South Korea's Jeju Island.

The East China Sea is bounded in the east and southeast by the middle portion of the first island chain off the eastern Eurasian continental mainland, including the Japanese island of Kyushu and the Ryukyu Islands, and in the south by the island of Taiwan. It connects with the Sea of Japan in the northeast through the Korea Strait, the South China Sea in the southwest via the Taiwan Strait, and the Philippine Sea in the southeast via gaps between the various Ryukyu Islands (e.g. Tokara Strait and Miyako Strait).

Most of the East China Sea is shallow, with almost three-fourths of it being less than 200 metres (660 ft) deep, its average depth being 350 metres (1,150 ft), while the maximum depth, reached in the Okinawa Trough, is 2,716 metres (8,911 ft).

China, Japan, the Korean peninsula and Taiwan lie within or border the East China Sea.